Voltage regulators for integrated circuits provide constant voltages to loads where the constant voltages are less than that of a common voltage, typically derived from a battery or other power supply, termed Vcc. Ordinarily the constant voltage, adjusted by voltage dropping circuits or resistors, is sufficient for most chip needs, except when much higher voltages are required, such as for programming EEPROM memory chips, where the programming voltage, Vpp, can be many times Vcc. In this situation a charge pump is used to boost Vcc to the Vpp level.
There are two major types of voltage regulators. A first type employs voltage sampling and comparison to a reference voltage. This type is commonly known as a feedback voltage regulator. A second type merely employs the reference voltage as part of a power supply circuit without comparison.
It has been realized in the prior art that a bandgap circuit is a useful tool for establishing the reference voltage, less than the power supply voltage Vcc. The bandgap circuit is combined with other circuit elements to derive desired regulated voltages. A bandgap voltage reference circuit relies on the basic physics of semiconductor materials to reliably establish a particular voltage. For example, in transistors, the bandgap voltage is closely related to a characteristic base-emitter voltage drop, Vbe, of a bipolar transistor. Many bandgap voltage reference circuits have been developed, one of which may be seen in U.S. Pat. No. 6,362,612 to L. Harris, which adapts the base-emitter characteristic of bipolar transistors to operate CMOS driver transistors.
Because bandgap circuits are well known in the art, they are commonly used as building blocks in more sophisticated voltage regulation circuits. For example, in U.S. Pat. No. 5,831,845 to S. Zhou, et al., it is shown how reference voltages, derived from bandgap voltage reference circuits, may be used to establish voltage regulation for an integrated circuit charge pump. S. Zhou, et al., explain that prior art voltage regulators use a pair of serially-connected capacitors of different sizes to achieve regulation. A first reference voltage is applied at a node between the two capacitors and a second reference voltage to a comparator, which controls the operation of the charge pump. The second reference voltage is slightly smaller than the first. There is sometimes a problem in the comparator incorrectly establishing the high voltage output and so S. Zhou, et al., provided an improved balanced capacitor voltage divider approach to voltage regulation for charge pumps.
As seen from the patent to S. Zhou, et al., several different voltages can be required. While most transistors are designed to operate at low voltage levels established from a regulated Vcc supply, EEPROM transistors require a programming voltage which is several times higher than Vcc, supplied from a charge pump. At the same time, since diverse voltage requirements appear at different regions of a chip, a chip-wide approach is needed for supplying these requirements without constructing a multiplicity of voltage regulators at various locations on a chip for different needs. However, in circuits such as charge pumps, involving rapid switching, voltage regulators may experience difficult operating conditions. When there is an abrupt current demand from a switch, voltage will initially drop until the regulator has time to compensate. With many switches all making near simultaneous start-stop current demands, a voltage regulator may become unstable and unable to provide a reliable supply to an entire chip.
An object of the invention was to provide a versatile, yet stable, voltage regulator for an integrated circuit that would also supply constant voltages for diverse circuit needs, even where high speed switching is involved.